This invention relates generally to electron exposure systems utilized to expose, treat and process coatings and materials. More specifically, the invention relates to a cold cathode gas discharge electron source having a broad uniform emitting area.
A requirement exists for a large-area electron beam source which is controllable, uniform, insensitive to poor vacuum and is long lived. There have been many processes developed in treating materials and in semiconductor device processing requiring electron beam broad area exposure. Many of these processes require an electron beam source which can be controlled in voltage and current, and can withstand the outgassing of the material being irradiated. Electron beam sources used to treat large volumes of materials have typically been isolated by means of a vacuum window to protect the electron emissive cathode from outgassing from the material being treated. Due to the difficulty of making large-area electron permeable vacuum windows, these systems have utilized a small scanning beam which is then raster scanned to uniformly expose larger areas. To achieve higher throughput processing it is advantageous to use a large-area or flood beam electron source, to expose the whole substrate simultaneously. However, without a vacuum window, these larger electron sources must be impervious to the outgassing of the materials being treated.
Many large-area thermionic cathodes have been developed. However sources of this type require a good vacuum environment to be long lived, and also generate a lot of heat, which can adversely affect the material being processed. For processes in which the substrate is temperature sensitive, a cold cathode is desirable. Several cold cathode electron sources have been devised. An electron beam system utilizing a large-area gas discharge source has been described by Rocca in U.S. Pat. No. 4,496,449. Another large-area electron exposure system (utilizing a cold cathode) is Hiroaka, U.S. Pat. No. 4,119,688 in which is described a pulsed glow discharge to illuminate a shadow mask. However, in pulsed glow discharges it is difficult to attain uniform exposure and precise dose control. Conventional glow discharge sources require a ballast resistor and are highly dependent on the vacuum operating pressure. Another large-area cathode, based on photoemission, is disclosed in U.S. Pat. No. 4,554,458, and utilizes an unpatterned photocathode to illuminate a shadow mask. However, photocathodes are easily poisoned and require an ultraclean high vacuum environment.
In conventional glow discharge processes the accelerating voltage and pressure are adjusted to control the discharge. One cannot achieve a large change in accelerating voltage without significantly altering the current or adjusting the pressure to keep the discharge stable, or both. If the target substrate outgasses, the pressure rises and the beam current rises uncontrollably. In U.S. Pat. No. 3,852,596, this problem is overcome by locating the cathode remotely from the target being irradiated and protecting the cathode by differential pumping the two enclosures. Differential pumping is achieved by physically separating the two volumes except for a small aperture through which the beam passes. The aperture is made small enough to limit the vacuum conductance such that a large pressure differential may be maintained between the two. This technique cannot be used for a large-area electron beam, which requires that the space between the cathode and target be open and relatively unimpeded.
In another prior art embodiment utilizing differential pumping to control a glow discharge electron source, Induni (Helv. Phys. Acta 20, 463, 1947) devised a means for producing a mostly monoenergetic electron beam. Induni separated the ionization and accelerating regions of a gas discharge electron source by enclosing the cathode in a tube with a small-aperture anode located close to the cathode. The small aperture anode is placed at a distance less than the mean free path of the electrons (which are accelerated toward the anode). Therefore no ionization takes place in the accelerating region. Electrons that pass through the anode aperture enter a larger chamber and create ions in this field free space. Those ions that happen to drift close enough to the small anode aperture are pulled into the accelerating field protruding through the aperture. The electron emission is controlled by adjusting the pressure in the field free space. Since a small aperture is used, the vacuum (pressure) levels can differ between the accelerating and ionization regions. This allowed a high vacuum to be maintained in the accelerating field region, thereby reducing the susceptibility of high voltage breakdown for high electron energy operation. Although Induni's electron gun solved many of the problems of a gas discharge electron source it produces a very small diameter and small angular beam which is not suitable for rapidly exposing large-area targets. Another disadvantage of this particular source is that to change the emission current requires a change in the pressure in the vacuum vessel (ionization region). To achieve a large change in emission current requires a large change in pressure. This is a serious drawback because it is very difficult to vary or control the pressure in a vacuum system rapidly and precisely. Another drawback of Induni's cathode is that it is designed to operate at a fixed accelerating voltage, whereas, in some processes, it is desirable to be able to vary continuously the accelerating voltage and consequently the energy of the electron beam incident on the target. In the Induni device this would not be possible without a large and precisely controlled change in pressure to maintain a constant emission current.
It will be appreciated from the foregoing that there is a significant need for an improved electron source that will overcome many of the disadvantages of the aforementioned earlier electron sources. In particular, what is needed is a broad area, uniform, cold, electron beam source with continuously variable voltage, capable of operation in a soft vacuum. Ideally, the electron source should also have a continuously variable accelerating voltage that can be utilized to rapidly process large-area substrates in semiconductor device fabrication for high resolution shadow mask lithography, photoresist curing or hardening, blanket exposure of resists for aiding liftoff processes, enhancing resist contrast or controlling resist pattern dimensions or pattern edge profiles. One object of this invention is to provide a continuously variable beam voltage from 1 to 30 KeV (thousand electron-volts) while maintaining the vacuum pressure within an easily controllable and narrow range.
Another object of the invention is to provide an improved cold cathode gas discharge source with a large emission area; for example, one to eight inches vide an electron source that can be controlled rapidly and accurately with an easily adjustable low bias voltage; for example, from 0 to 10 volts, to provide a device that is continuously variable in accelerating voltage without requiring large variations in operating vacuum level. It is a further object of the invention to provide a large-area cathode with substantially uniform emission over its entire surface and whose emitting surface does not contaminate, become poisoned, burn up, or oxidize, and is generally impervious to gas species that might be outgassed by the target being irradiated by the electron beam. Finally, it is also an object of the invention to provide a broad beam electron source which will operate in a continuous, rather than pulsed, emission mode, in a poor or soft vacuum environment and in close proximity to the target it is irradiating.
All of these objects are believed to be provided by the present invention, which is next described in summary form.